I had a 10 hour layover in Lima just last May. Some things to know. There is luggage storage right after the baggage claim. It took a long time to get downtown. I arranged a half day tour with a group from the airport in advanced. We were scheduled to see 8 or so sights and had to cut a few due to traffic. I still got a good vibe of the city and there were some interesting areas but it was really rushed. I believe my layover was from noon till 10pm? So maybe traffic isn't as bad earlier in the day but I doubt it.

As far as is it worth giving up a day in Cusco for 1 in Lima. I wouldn't go into Lima unless I had at least 1 full day to cover the sights. How many days are you going to be in Cusco? I loved Cusco and thought it was a great, diverse city. I spent a lot of time checking out all the cathedrials and going to the farmers market. Don't get me wrong Lima had some highlights and I don't regret even my rushed visit but a full day would be needed to do it justice and if you are in Cusco for limted time it would best be spent doing Cusco right.